A user device, which can be wireless or wireline, might be in an access network or domain in which the Gm interface (e.g., 3GPP TS 23.292), which provides direct communication to a home carrier's Internet Protocol Multimedia Subsystem (IMS) domain, is not available or is not desirable for use. In this situation, IMS-related call control might not be supported. Currently, in 3GPP (3rd Generation Partnership Project) specifications (e.g., TS 23.292), I1 and I2 ICS (IMS Centralized Services) reference points have been defined to solve this problem. However, the I1 implementation requires significant development on the user device (e.g., handset) and the I2 implementation requires significant investment in Mobile Switching Center (MSC) based technology. Further, neither the I1 solution nor the I2 solution is attractive considering the evolving nature of the problem. For example, as a Long Term Evolution (LTE) network expands, there will be fewer and fewer circumstances in which user devices are on access networks that do not provide direct communication to the IMS domain. The I2 method can also depend on a visited (serving) carrier's willingness to upgrade their MSCs.
The 3GPP specification (e.g., TS 23.292, Sec 5.3.1) has defined the IMS SCC-AS (Service Centralization and Continuity Application Server) to be the last network entity in the terminating iFC (initial Filter Criteria) chain of application servers. This design assumes that IMS control is desired for all calls. However, in some cases, it is desirable to use the non-IMS domain to perform voice call control. For example, when a user roams on a non-IMS foreign network that may not support the home carrier's advance services, it might be desirable to use the non-IMS domain for voice call control.